In a light water reactor, a reactor internal structure installed on a bottom of a reactor pressure vessel is generally formed of a material having an excellent corrosion resistance and a high-temperature strength, such as an austenitic stainless steel and a high nickel alloy. However, even a reactor internal structure formed of such a material may suffer from a material deterioration which is caused by a lengthy operation under a high temperature and a high pressure and by an irradiation of neutron. In particular, in a portion near a welding part of a reactor internal structure, a material thereof may be liable to cause a crack or a tensile residual stress may be generated, because of a heat generated upon welding. In this case, there is a possibility that a stress corrosion cracking occurs.
Laser peening is known as one of maintenance techniques for preventing the stress corrosion cracking, and various methods thereof have been developed. In the laser peening, a pulse laser is irradiated onto a material to generate a plasma on a surface of the material, and a kinetic energy of an impulse wave of the plasma is utilized to change a tensile residual stress on the material surface into a compressive stress. Thus, there is eliminated a stress factor (tensile residual stress caused by welding), which is one of three factors inviting the stress corrosion cracking, i.e., a material factor, an environmental factor, and the stress factor. Accordingly, the stress corrosion cracking in a portion near a welding part can be prevented.
For example, JP2002-328193A (see, Patent Document 1), JP2001-255395A (see, Patent Document 2), and JP2005-227218A (see, Patent Document 3) respectively propose an apparatus that subjects a reactor internal structure to a laser peening process (laser peening apparatus), with a view to preventing the stress corrosion cracking. This kind of laser peening apparatus has a vertically longer length and a large mass. In order to subject a reactor internal structure to a laser peening process during a maintenance/repair operation of the reactor internal structure, it is necessary to precisely arrange a laser peening apparatus in position, and to stably fix the same. To this end, a lower part of the laser peening apparatus has to be fixed on the reactor internal structure, such as a control-rod drive mechanism housing or an in-core instrumentation cylinder, which is installed on a bottom of a reactor pressure vessel, while an upper part of the laser peening apparatus has to be held by a reactor internal support, such as an upper lattice plate or a core support plate, which is disposed on an upper part of the reactor pressure vessel (see, FIG. 17).
However, when the reactor internal structure such as the upper lattice plate or the core support plate is not disposed, for example, during a work in which the reactor internal support is replaced, the upper part of the laser peening apparatus cannot be held. In this case, it is difficult to stably fix the laser peening apparatus inside the reactor pressure vessel. Therefore, when the reactor internal support is not disposed, it is impossible to maintain/repair the reactor internal structure by using the laser peening apparatus.